Effects of Substitution Site on Acetyl Amylose Biodegradability by Amylase Enzymes

The site-selective syntheses of water soluble (6-O)- and (2-O/3-O)-acetyl amylose polymers (substituted at primary and secondary hydroxyl functionalities, respectively) were carried out. On the basis of 1H NMR analyses regiospecificities of >95% were achieved. In addition, routine chemical methods which did not employ protection−deprotection steps provided water soluble (2-O/3-O/6-O)-acetyl amylose polymers. To maintain water solubility, the polymer degree of substitution (ds) was maintained at <0.70. The biodegradation characteristics of these products as a function of site and ds were studied by exposures to the α-amylases from Bacillus subtilis, Bacillus licheniformis, and Aspergillus oryzae. Quantitation of the biodegradation rate and percent were carried out using the dinitrosalicylic acid (DNS) reducing sugar assay. Common to all three α-amylases was that these enzymes degraded (2-O/3-O)-acetyl amylose polymers much more rapidly and to greater extents than (6-O)-acetyl amylose derivatives of similar ds's and molecular weights (Mv). The rate of and percent degradation of (2-O/3-O/6-O)-acetyl amylose polymers was intermediate to that of (2-O/3-O)- and (6-O)-acetyl amylose polymers. Thus, the importance of site of substitution on the biodegradability of acetyl amylose polymers was demonstrated. Interestingly, when low ds (∼0.20) acetyl amylose polymers were exposed to the exoglycosidase from sweet potatoes (β-amylase), little to no polymer degradation was observed. This is believed to result from the rapid formation of substituted chain ends that are not degraded by the β-amylase, thus terminating further chain degradation events.